In the welding industry, a principal characteristic desired or demanded of weld beads is high tensile strength and ductility, coupled with a toughness as measured by the well known Charpy V-Notch Impact Test and now more recently by a Crack Tip Opening Displacement (CTOD) test. In the Charpy Test, a chilled specimen of prescribed dimensions is placed in a test machine and the energy required to fracture the specimen on impact is then measured. In the CTOD test, a fatigue crack is developed, then the specimen is subjected to stress until it breaks. The higher the energy or CTOD value, the better the weld. Only the Charpy test will be referred to hereinafter.
In the welding of heavy steel plates, it is conventional to cut samples from the top, bottom and middle of a sample weld, and to measure the impact strength of each of these sections.
In the welding of heavy steel plates, on the order of one and one-half to two inches and more, it is conventional to: bevel the edges of the plates; place the sharp edges thus formed in close spaced relationship; weld the root together; and then with a series of overlapping arc weld passes, fill up the V-notch formed by the bevelled edges. In such an operation, each successive weld pass partially melts the previously deposited weld bead and reheats the metal adjacent to the re-melted metal to a temperature above its transformation temperature.
In such multiple pass welding, heretofore using the same electrode, it has been difficult to obtain Charpy Impact values over the entire depth of the weld which were acceptable to industry.
The desired result has been accomplished by changing the electrode employed for successive passes, but this makes the operation complicated, time-consuming, and expensive. The present invention obtains the high Charpy Impact Test values required using a single electrode.
Another problem has been that in order to obtain the high impact strengths desired in multi-pass welding, it has been necessary to impose limitations on the size of the weld deposit which could be laid down in each pass or layer. The size of the weld deposit is determined by the electrode size, the electrode feed speed, and the travel speed. Thus, by limiting the size of each deposit, a greater number of passes were required to fill the joint, which resulted in a longer time to complete the total weld.
Correlative to this, was that the welding parameters had to be established and the welding operator made to comply to these parameters. If he exceeded them, weld beads which did not have the desired Charpy Impact strength resulted.
Using the present invention, the number of passes is substantially reduced and there is no limit on the size of each layer.
Another problem, because of the limitation on the thickness of each deposit laid down, was that the operators were required, when welding vertically extending butt joints, to weld from the top down. Using the present invention, it is now possible to obtain weld beads having the desired impact strengths by welding vertically up, a less costly procedure.
A still further problem has been variation in the impact strength from the root to the cap on the weld bead. This may be explained by the fact that electrodes normally include titanium in some form in the flux, which titanium ends up as a residual in the weld bead. Some titanium is necessary to provide the desired impact values. However, titanium in excessive amounts is detrimental to the impact value.
As a result, the electrode had to contain enough titanium so that in the root pass(when there is substantial dilution of the electrode metal by the metal of the workpiece which melts and becomes part of the weld bead), there will be sufficient titanium to provide the desired impact value. However, as the subsequent layers are deposited, there is less and less dilution from the metal of the workpiece and, ultimately, the amount of titanium in the weld bead reaches a value where the impact value begins to decrease. Also, the final or capping layer is never reheated as with the root pass or intermediate passes and thus does not receive the grain refining effects of this reheating. Thus, the weld bead analysis of this final capping pass is important.
As will appear using the present invention, there is an increase in the titanium residual in the weld bead, but the maximum value reached is less than the critical value where impact values begin to decrease with increased titanium content. Also, titanium content in the final or capping pass is low enough as not to require any grain refinement by reheating.
In the past, in the arc welding of steel, fluxes have been used either on the inside of a tube, coated on an electrode wire, or as a pile of granular flux on the weld bead, for various purposes including: (1) to add alloying elements to the weld bead, but (2) first and foremost, to exclude or limit nitrogen from the metals transferring from the electrode wire to the weld pool or from the weld pool itself. At the temperature of the arc, molecular nitrogen from the atmosphere tends to decompose and is carried into the molten weld puddle. Then, as the weld puddle cools, the atomic nitrogen returns to the molecular form and is released as nitrogen gas within the weld bead resulting in porosity and a defective weld bead.
Thus, using a bare steel wire and no shielding gas, the fluxing ingredients always included compounds which would vaporize in the heat of the arc to exclude nitrogen from the vicinity of the arc. In the alternative, shielding gases coaxial with the electrode wire and the arc, were employed to exclude nitrogen. Granular flux deposited on top of the weld bead through which a bare electrode wire is advanced, has also been employed.
As will appear, to some extent the present invention reverses this objective of the prior art, and makes beneficial use of small amounts of nitrogen in the weld metal but as a compound of titanium.